Isolated pixel liquid crystal light valve structure

ABSTRACT

A photoconductive substrate is provided to voltage modulate a liquid crystal layer in response to input light. The substrate is partitioned into electrically isolated pixels to eliminate lateral spread of charge carriers therein, and increase the dynamic range of the liquid crystal light valve while preserving resolution. The substrate is partitioned by forming an interconnecting network of deep trenches in a surface thereof, and filling the trenches with an insulating material such as silicon dioxide. The opposite surface of the substrate is etched away to expose the silicon dioxide in the trenches, thereby providing the substrate with partitions which extend completely therethrough between the opposite surfaces.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the field of optoelectronics,and more specifically to an isolated pixel photoconductive structure forvoltage modulating a liquid crystal layer in a liquid crystal lightvalve.

2. Description of the Related Art

The silicon photoconductor based liquid crystal light valve (LCLV), orspatial light modulator, performs the function of converting an inputlight image having a certain wavelength, intensity, and coherenceconditions into an output image in which some or all of these parametersare varied. Applications of LCLVs include image amplifiers, wavelengthconverters, incoherent-to-coherent image converters, and adaptiveoptics. While image amplifiers find uses in large screen displays suchas theaters, flight simulators, and command and control displays, imagewavelength converters are used for displaying visible images frominfrared scenery and the like. Incoherent-to-coherent image convertersare used primarily for optical image processing.

The silicon LCLV to which the present invention constitutes a novelimprovement is described in an article by U. Efron et al, entitled "Thesilicon liquid-crystal light valve", J. Appl. Phys. 57(4), Feb. 1985.The device consists of a high resistivity, π-silicon photoconductivelayer or substrate, coupled with a dielectric silicon dioxide layer toform an MOS structure. A unified thin-film structure consisting of adielectric mirror and a light blocking layer provides the high broadbandreflectivity required, as well as optical isolation of thephotoconductor from the high-intensity readout beam. The readout beam isreflected by the dielectric mirror through the liquid crystal. Thelatter is usually operated in a hybrid field effect mode. The MOS modeof operation consists of periodic depletion and accumulation phases. Inthe depletion (active) phase, the high-resistivity π-silicon is depletedcompletely, and electron-hole pairs generated by the input light areswept by the electric field, thereby producing the signal current thatactivates the liquid crystal.

The electric field existing in the depletion region acts to focus thesignal charges, and to preserve the spatial resolution of the inputimage at relatively low photogenerated charge densities. However, atlarger charge densities associated with a larger signal dynamic range,the photogenerated charge carriers will diffuse or spread laterally in apotential free region near the silicon back surface where the charge isgenerated. This is caused by partial collapse of the depletion region asthe charge density increases. The signal charge goes through furtherlateral drift and diffusion as it drifts through the silicon thickness,and most importantly, to an even greater extent at thesilicon/dielectric layer interface where the charge resides for a finitelength of time. This lateral drift and diffusion of the signal chargeresults in significant loss in device resolution.

The lateral spread of photogenerated charge at the silicon/dielectricinterface has been conventionally limited by means of a grid of"microdiodes" formed by regions of opposite doping polarity implantedinto the silicon layer at the interface. The grid acts to focus theincoming charge carriers into the resolution cell defined by it, as wellas to form "charge buckets" of carriers already residing at theinterface.

Although effective at relatively low levels of photogenerated charge,the microdiode grid cannot contain the signal charge residing at theinterface at high excitation levels. Once the potential wells formed bythese diodes are partially filled with the signal charge, the surfacepotential is decreased and the charge can spill over to adjacent pixelareas. In addition, the microdiodes are not operative to prevent lateralspread of charge carriers at the silicon back interface at which thecharges are generated, or in the bulk portion of the silicon layer. Thislimits the liquid crystal voltage swing and thereby the signal dynamicrange and image contrast attainable in a LCLV without an accompanyingloss of resolution.

SUMMARY OF THE INVENTION

The main purpose of the present invention is to improve the resolution,contrast ratio, and dynamic range of a LCLV. This is accomplished byelectrically isolating each pixel or picture element from neighboringpixels by means of electrically insulating partitions formed through thethickness of the silicon photoconductive substrate. In this manner, thephotogenerated charge is positively prevented from interdiffusionbetween adjacent pixels, even at very high levels of charge densitywhich were unattainable in the prior art. The result is that the dynamicrange and contrast of the image formed by voltage modulation of theliquid crystal layer by the partitioned silicon photoconductive layer ismuch higher than has been possible heretofore, while maintaining highimage resolution.

The above purpose is achieved in accordance with the present inventionby providing a pixelized photoconductive substrate for voltagemodulating a liquid crystal layer in response to input light. Thesubstrate is partitioned into electrically isolated pixels to eliminatelateral spread of charge carriers therein, and increase the dynamicrange of the liquid crystal light valve while preserving resolution. Thesubstrate is partitioned by forming an interconnecting network of deeptrenches in a surface thereof by, for example, magnetron plasma etching,and filling the trenches with an electrically insulating material suchas silicon dioxide. In the preferred embodiment of the invention, theopposite surface of the substrate is etched away to expose the silicondioxide in the trenches, thereby providing the substrate with partitionswhich extend completely therethrough between the opposite surfaces. Ifdesired, the insulator-filled trenches may extend only partially throughthe substrate, thereby eliminating lateral spread of charge particles inthe thickness of the substrate which is partitioned by the trenches.

These and other features and advantages of the present invention will beapparent to those skilled in the art from the following detaileddescription, taken together with the accompanying drawings, in whichlike reference numerals refer to like parts.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a silicon LCLV embodying thepresent invention;

FIG. 2 is a fragmentary plan view illustrating a partition arrangementfor isolating pixels in a photoconductive substrate of the present LCLV;

FIGS. 3a to 3f are sectional views illustrating a method of fabricatingan isolated pixel photoconductive substrate in accordance with thepresent invention; and

FIG. 4 is a sectional view illustrating an alternative isolated pixel,photoconductive substrate structure embodying the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 of the drawings, a preferred embodiment of a siliconLCLV according to the present invention is generally designated as 10and includes a voltage modulating photo-substrate 12 having a highresistivity, π-type silicon layer 14 With reference also being made toFIG. 2, the substrate 12 is partitioned into a plurality of electricallyisolated pixels 16 by an interconnected network of channels 18 filledwith an electrically insulating material 20. The channels 18 arepreferably in the form of a rectangular grid.

A layer 22 of a very low resistivity P⁺ silicon is formed on a firstsurface 24 of the substrate 12, for example by ion implantation. Thechannels 18 extend through and partition the layer 22 in the same manneras the layer 14. A common electrode layer 26 of deposited lowresistivity polysilicon or the like is formed over the first surface 24of the substrate 12 in electrical contact with the areas of theconductive layer 22 between the channels 18.

A transparent face plate 28 is adhered to the electrode layer 26 bymeans of an appropriate cement (not shown) or electrostatic bondingthrough the intermediary of

a trilayer 30 including a SiO₂ layer 32, a Si₃ N₄ layer 34, and a SiO₂layer 36. The material of the plate 28, preferably a suitablecomposition of glass, is selected to be thermally matched to the siliconlayer 14. The trilayer 30 serves as a mobile ion barrier between theplate 28 and silicon layer 14.

The present LCLV 10 further includes a dielectric layer 38, formed of anappropriate, electrically insulating material such as SiO₂, on a secondsurface 40 of the substrate 12. A light blocking, dielectric mirrorlayer 42 is formed on the dielectric layer 38. A liquid crystal layer 44is formed on the dielectric mirror layer 42. A transparent face plate 48is coated with a transparent electrode layer 46 to form a sandwich withthe other layers of the LCLV 10 together in combination with the faceplate 28. Further illustrated is a guard ring 50 formed of n-silicon inthe second surface 40 of the substrate 12 around the periphery thereof,which prevents peripheral minority carrier injection into the activeregion.

The LCLV 10 has the overall structure of a metal oxide semiconductor(MOS) capacitor, including the transparent electrode layer 46, theliquid crystal 44, dielectric mirror 42, and the dielectric oxide 38layers as the overall insulator, and semiconductive silicon layer 14.The LCLV 10 is operated in alternating accumulation and depletion phasesin response to an alternating current voltage applied from a powersource 52 connected across the electrode layers 26 and 46. Thealternating current waveform applied by the source 52 is in the form ofa pulse having a relatively short negative interval corresponding to theaccumulation phase and a relatively long positive interval correspondingto the depletion phase. Further illustrated is a direct current powersource 54 for biasing the guard ring 50 positive with respect to thefirst electrode layer 26.

During the accumulation phase, a short negative pulse is applied whichrenders the second electrode layer 46 negative with respect to the firstelectrode layer 26. This causes the MOS capacitor to be charged by meansof majority carriers (holes for the positive π-silicon layer 14)accumulating at the second surface 40 which constitutes an Si/SiO₂interface. The minority carriers (electrons) residing in the interface(from a previous depletion phase) are pushed through the layer 14 towardthe first electrode layer 26 and recombine with the majority carriers inthe bulk portion of the layer 14. On the other side of the dielectriclayer 38, electrons are pushed from the second electrode layer 46through the liquid crystal layer 44 toward the dielectric layer 38.

In the depletion phase, which constitutes the main, active phase, thesecond electrode layer 46 is made positive with respect to the firstelectrode layer 26. The majority carriers (holes) are pushed away fromthe dielectric layer 38 toward the first electrode layer 26, creating adepletion region throughout the silicon layer 14. During this time,minority charge carriers, which are electrons in this case, are releasedfrom the electrode layer 26 and P⁺ doped silicon layer 22 and injectedinto the silicon layer 14.

A light image generated by an optical imaging system, cathode ray tube,or the like (not shown) is focussed onto the LCLV 10 through the faceplate 28 in the direction of an arrow 56. Light incident on the siliconlayer 14 causes localized absorption and the liberation of electron-holepairs. Photogenerated electrons are swept by the electric field throughthe silicon layer 14 toward the dielectric layer 38. As a result, thesilicon surface potential is modulated in response to the sweptphotogenerated electrons. This in turn results in a correspondingmodulation in the voltage drop across the liquid crystal layer 44, henceits local activation. The activated regions of the liquid crystal layer44, corresponding to the photoconductive regions in the silicon layer44, modulate a readout light beam which is reflected by the dielectricmirror layer 42 as symbolically represented by an arrow 58. Theoperation of the LCLV 10 is such that the liquid crystal layer 44 isvoltage modulated by the photoconductive silicon layer 14 of thesubstrate 12.

The problem which has remained in the prior art is that the minoritycharge carriers, in this case electrons, which are photogenerated in theπ-silicon layer 14 in the vicinity of the first surface 24, tend tospread laterally due to diffusion as they are swept toward the secondsurface 40 by the applied electric field during the depletion phase ofoperation. This lateral diffusion begins in the depleted region near thesurface 24, continues through the bulk of the layer 14, and isespecially prevalent at the interface with the dielectric layer 38 wherethe charge resides for a finite length of time. As discussed above, theprior art expedient of a microdiode grid provided at the interface ofthe layers 14 and 38 is insufficient to prevent lateral diffusion ofelectrons at high charge densities, and is inoperative to preventlateral diffusion in the bulk of the layer 14.

The LCLV 10 prevents lateral diffusion in the voltage modulating,photoconductive substrate 12 at all values of charge density byproviding a partition means in the form of the channels 18 filled withan electrically insulating material 20 such as silicon dioxide, topartition the layer 14 into electrically isolated pixels. The partitionsextend completely through the thickness of the layer 14 from the firstsurface 24 to the second surface 40 thereof, and also completely throughthe conductive layer 22. The insulative material 20 acts as a totalbarrier for lateral charge carrier diffusion, and thereby preventsinteraction between adjacent pixels and a resulting loss of resolution.Thus, the dynamic range of the LCLV 10 and thereby the contrast ratioattainable may be increased to a level which has not been possibleheretofore, while maintaining high device resolution.

A method of fabricating a LCLV including an isolated pixel, lightmodulating, photoconductive substrate in accordance with the presentinvention is illustrated in FIGS. 3a to 3f, in which elements subject tosubsequent processing steps are designated by the same referencenumerals used in FIGS. 1 and 2 primed. In FIG. 3a, a substrate 12' isprovided in the form of a π-type silicon wafer approximately 5 milsthick, having an electrical resistivity on the order of 2 to 5 ohm·cm. AP⁺ silicon layer 22', which may be only a few tenths of a micron thick,is formed on the substrate 12' by ion implantation or any other suitablemethod. The layer 22' is heavily doped with boron or the like to make ithighly conductive.

Referring now to FIG. 3b, deep trenches 18' are formed in the substrate12' through the layer 22' by magnetron plasma etching or any othersuitable process, using, for example, an aluminum mask to preventetching in the areas between the trenches 18'. The trenches 18' aretypically 2 to 3 microns wide and 20 to 60 microns deep. The spacingbetween adjacent trenches 18' is approximately 20 microns. Afterformation of the trenches 18', an optional passivation layer 60 ofthermal oxide may be grown on the side walls of the trenches 18' toanneal any possible damage from the plasma etching, and to passivate thesurface states.

It will be understood that the spacing between adjacent trenches 18' andthe width of the trenches are made as small as possible in order tomaximize image resolution while maintaining electrical isolation betweenpixels. The smallest values of trench width and spacing attainable inactual practice are limited by the level of existing manufacturingtechnology and realistic fabrication cost constraints. The values givenabove should be considered as being exemplary, and not restrictive ofthe scope of the invention.

The next step of the method is illustrated in FIG. 3c, and consists ofdepositing a layer 20' of SiO₂ over the substrate 12' which fills thetrenches 18' and forms a coating on the substrate 12'. The layer 20' isthen etched back as shown in FIG. 3d to a sufficient depth so as toexpose the insulative material 20 in the trenches 18' and the conductivelayer 22 between the trenches.

FIG. 3e illustrates the formation of the first electrode layer 26 on thesubstrate 12. The layer 26 may be formed by deposition or any othersuitable method, and is then preferably planarized using a standardphotoresist etch-back technique. As an alternative to the deposited P⁺polysilicon layer as discussed above, the layer 26 may be formed of ITO.It is necessary that the layer 26, which constitutes an electrode, be inintimate electrical contact with the silicon layer 14 of the substrate12 through the conductive layer 22.

In FIG. 3f, the trilayer 30 and face plate 28 are electrostaticallybonded to the substrate 12'. The plate 28 is thermally matched to thesilicon photosubstrate 26. Diffusion of mobile ions in the glass 28 tothe photosubstrate plate 28 is prevented by the trilayer 30.

As further illustrated in FIG. 3f, a sufficient thickness of thesubstrate 12' below the trenches 18' is etched or otherwise removed toexpose the insulative material 20 in the trenches 18'. The exposedsecond surface of the substrate 12 is polished using standard lappingand chemomechanical polishing techniques. The thickness of the substrate12 after etching and polishing is preferably 30 to 100 microns. Thiscompletes the formation of the partitions in the substrate 12. Afterthis step, the exposed trenches 18' constitute the channels 18illustrated in FIG. 1.

Following the steps of FIG. 3f, the dielectric layer 38, dielectricmirror layer 42, are deposited onto the second surface 40 of thesubstrate 12 using conventional plasma enhanced chemical vapordeposition (PECVD) gate oxide deposition and annealing, and dielectricmirror deposition techniques. Fabrication of the isolated pixel LCLV iscompleted by insertion of the liquid crystal layer 44 between thedielectric mirror layer 42 and the face plate 48 coated with the secondelectrode layer 46.

The result of these final steps is the finished LCLV 10 illustrated inFIG. 1. The dielectric mirror layer 44 may be a Si/SiO₂quarter-wavelength stack. The liquid crystal layer 44 may be formed of apositive anisotropy, 45° twisted-nematic, or perpendicular-alignedliquid crystal material.

It will be noted that the process steps of the invention need notnecessarily be performed in the order illustrated in FIGS. 3a to 3ewithin the broad concept of the invention. For example, the steps ofremoving material 10 from the second surface 40 of the substrate 12' toexpose the insulative material 20 in the trenches 18' as illustrated inFIG. 3f and the subsequently described steps may be performed prior tothe steps beginning with FIG. 3c. This is because the processing of oneside of the substrate 12' subsequent to formation of the trenches 18'and the filling thereof with the insulative material 20' may beperformed independently of the processing of the other side of thesubstrate 12'.

The present isolated pixel, photoconductive substrate may beincorporated into any applicable type of LCLV other than the particulartype described above, such as a metal matrix mirror IR-LCLV. Asalternatives to silicon, the photoconductive substrate may be formed ofany suitable material, such as gallium arsenide or cadmium sulfide. Thematerial utilized to fill the partition trenches may be any appropriateelectrically insulating material other than silicon dioxide, such assilicon nitride or a polymer substance. Although the preferred method offorming the trenches is magnetron plasma etching, any other processwhich accomplishes the desired result, such as wet etching or laseretching may be employed.

It is within the scope of the invention to form the trenches in thesecond surface of the substrate rather than the first surface thereof.Another modification is to provide electrically insulating partitions 72which extend only partially through the thickness of a photoconductivesubstrate 70, as illustrated in FIG. 4. In this latter embodiment, thereduced level of electrical isolation between pixels may be justified bya lower processing cost.

As yet another modification, the conductive layer 22 may be omitted andthe electrode layer 26 bonded directly to the silicon layer 14.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art, without departing from the spirit and scopeof the invention. For example, the materials, relative electricalpolarities and semiconductor impurity doping levels, etc. may bereversed or varied within a considerable range without departing fromthe teachings of the present disclosure. Accordingly, it is intendedthat the present invention not be limited solely to the specificallydescribed illustrative embodiments. Various modifications arecontemplated and can be made without departing from the spirit and scopeof the invention as defined by the appended claims.

I claim:
 1. A photoconductive structure for a liquid crystal lightvalve, comprising:a photoconductive, voltage modulating substrate; andelectrically insulative partition means extending through the substratebetween opposite surfaces thereof for partitioning the substrate into aplurality of electrically isolated pixels, said partition meanscomprising an interconnected network of channels filled with anelectrically insulating and optically transparent material.
 2. Aphotoconductive structure as in claim 1, in which the substrate isformed of a semiconductive material.
 3. A photoconductive structure asin claim 2, in which the semiconductive material is π-type silicon.
 4. Aphotoconductive structure as in claim 1, in which the substratecomprises semiconducting silicon and the insulating material comprisessilicon dioxide.
 5. A photoconductive structure as in claim 1, in whichthe channels are arranged in a rectangular grid with predeterminedchannel widths and spacings between adjacent channels, the spacingbetween adjacent channels is selected to enhance image resolutioncompared to said photoconductive structure without said channels, andthe width of each channel is selected to maintain electrical isolationbetween pixels.
 6. A photoconductive structure as in claim 5, in whichthe channels each have a width of 2 to 3 microns.
 7. A photoconductivestructure as in claim 1, further comprising a mobile ion barrier on oneside of said substrate, said barrier including layers of the samematerial as said partitions.
 8. A photoconductive structure as in claim7 wherein said material is silicon dioxide.
 9. A liquid crystal lightvalve, comprising:a liquid crystal layer; a photoconductive layer forvoltage modulating the liquid crystal layer in response to input light;and electrically insulative and optically transparent partition meansextending through the photoconductive layer between opposite surfacesthereof for partitioning the photoconductive layer into a plurality ofelectrically isolated pixels, said partition means having aninterconnected network of channels filled with an electricallyinsulating material.
 10. A liquid crystal light valve as in claim 9,further comprising:first and second transparent electrodes whichsandwich the liquid crystal layer and photoconductive layertherebetween; a dielectric layer disposed between the liquid crystallayer and the photoconductive layer; and light reflecting and blockingmeans disposed between the liquid crystal layer and the dielectriclayer.
 11. A liquid crystal light valve as in claim 10, furthercomprising power source means for applying an alternating currentelectrical voltage across the first and second electrodes.
 12. A liquidcrystal light valve as in claim 9, in which the photoconductive layercomprises silicon and the insulating material comprises silicon dioxide.13. A liquid crystal light valve as in claim 12, in which said siliconis π-type silicon.
 14. A liquid crystal light valve as in claim 9, inwhich the photoconductive layer is formed of a semiconducting material.15. A liquid crystal light valve as in claim 9, further comprising amobile ion barrier on the opposite side of said photoconductive layerfrom said liquid crystal layer, said barrier including layers of thesame material as said partitions.
 16. A liquid crystal light valve as inclaim 15, wherein said material is silicon dioxide.
 17. A liquid crystallight valve as in claim 15, wherein said barrier is separated from saidsubstrate by a conductive electrode.